soc: imx: gpc: Use GENPD_FLAG_RPM_ALWAYS_ON for ERR009619

This allows PU domain to be turned off in suspend and save power.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>